1. Field of the Invention
The present invention relates generally to flat panel display devices and more particularly, to a phase transition flat panel display including Vanadium Dioxide (VO2)--based pixels in order to electrically modulate light by utilizing the phase transition property of VO2.
2. Description of the Prior Art
Display devices are conventionally classified into two basic categories including active and passive displays. Active displays which are light generating devices include such technologies as Cathode Ray Tubes (CRT), Light Emitting Diodes (LED) and Plasma Display Panels (PDP), while passive displays are light modulating devices where the light source is either ambient or light from a separate source and includes such technologies as Liquid Crystal Displays (LCD), Electrochromic Displays (ECD) and Electrophoretic Displays (EPID).
Another classification for displays relates to the physical size or geometry of the device. Flat Panel displays are generally more compact and energy efficient, and utilize practically all of the above mentioned technologies except for CRT technology. Attempts to flatten the conventional CRT have been unsuccessful since the devices produced have had either poor picture quality or excessive manufacturing costs.
A very successful type of Flat Panel Display is the LCD device. The LCD includes a plurality of pixels arranged in a matrix configuration utilized to either transmit or block light. Whether light is transmitted or blocked, depends on the alignment of the liquid crystal molecules which is controlled by an electrical bias. The early LCDs utilized a "passive matrix" scheme in order to address the individual pixels when producing images. This scheme consists of applying a voltage to a single row and then adjusting the column voltages to produce a large combined voltage across the selected pixels in that row. This addressing scheme enabled the early LCDs to be efficient and low cost. However, due to a cross talk complications, the Passive Matrix LCDs cannot provide both good contrast and resolution.
In order to overcome the cross talk problem, the "active matrix" scheme was developed for LCD devices. This scheme utilizes an array of transistors in order to address the individual pixels. Each pixel receives a voltage from its column line only when its own transistor is switched on. This enables Active Matrix LCDs to provide good resolution as well as good contrast. However, these devices have some drawbacks. First of all, these type of displays draw more power than a display utilizing the "passive matrix" scheme. These types of displays are also more expensive and complicated to produce. Another drawback is that these type of displays tend to have lower yields due to the difficulty of fabricating the transistor arrays which is needed to perform the "active matrix" addressing.
Another type of flat panel display is the ECD device. The ECD device generally includes a cell with at least two electrodes where at least one consists of electrochromic material, an electrolyte and at times an insulator. Applying a voltage across the electrodes causes ions present in the electrolyte to be absorbed by one of the electrodes thereby producing a change of color or transmissive property in the electrode. The change in color or transmissive property is the effect that enables these types of displays to produce images.
VO2 exhibits an insulator-to-metal phase transition at temperature T.sub.c =68.degree. C. which is accompanied by a significant change in electrical and optical properties. Due to this feature this material has been utilized in various electrical and optical applications. These applications have included: a medium for holographic optical recording, a temperature stabilizer and controller, an electronic switch, material for screening and modulating microwave radiation and electronic and optical memory elements. One of the optical properties which is significantly changed is the index of refraction, which would enable VO2 to modulate light reflectance. The phase transition in VO2 can be thermally induced by utilizing heater elements disposed under the film of this material. Some primitive display functions have been demonstrated earlier, with modulation of the reflectance of the VO2 with an external heater. The latter heated the glass substrate as well, causing a high power consumption and a long response time of 0.2-0.5 s.
It is therefore, an object of the present invention to provide an improved Flat Panel Display by employing phase transition of the pixelized VO2 film in a new embodiment which allows selective heating of the light modulating film and more efficient use of this material to provide new display functions and properties, such as video frequency of operation, high resolution, gray levels and color, together with a low dissipating power and utilizing well established manufacturing technique which provides high yield and lower cost.